羌塘盆地南部隆鄂尼地区布曲组鞍形白云石成因

羌塘盆地南部隆鄂尼地区中侏罗统布曲组砂糖状白云岩中发育有大量的鞍形白云石，其岩相学特征表现为正交偏光下的波状消光以及弯曲的晶面和解理，阴极发光整体呈暗红色，无明显的环带。鞍形白云石中流体包裹体均一温度主要分布于152.8～174.1℃，盐度均值（23.3% NaCl）远高于现代海水盐度，表明其形成经历了高温高盐度的演化过程。微区同位素分析显示鞍形白云石δ¹³C值介于-4.81‰～4.29‰，δ¹⁸O值为-11.2‰～-7.51‰，根据白云石-流体氧同位素分馏方程得到成岩流体δ¹⁸O （SMOW）值为5‰～11‰。综合分析认为，鞍形白云石形成于相对封闭的深埋藏环境，是热液流体调整改造作用的产物。地层水加热再循环过程中在孔隙或裂缝较为发育的部位沉淀形成鞍形白云石，高盐度的地层水可能来自沉积期古海水与埋藏期地下热卤水的混合。鞍形白云石的成因研究表明，晶粒相对较粗大的砂糖状白云岩是地表/近地表形成的白云岩重结晶作用的产物。

Origin of saddle dolomites from the Buqu Formation of Longeni Area in southern Qiangtang Basin

Saccharoidal dolostones in the Middle Jurrasic Buqu Formation of Longeni Area in southern Qiangtang Basin were found to contain large number of saddle dolomites.Petrographic study shows typical sweeping extinction,curved crystal faces and cleavage traces under cross polarized light and dark red luminescence and unzoned textures.Fluid inclusions in the saddle dolomite were measured to have homogenization temperature ranging between 152.8 and 174.1℃ and salinity averaged at 23.3%NaCl,much higher than the salinity of modern seawater,indicating a high temperature and high salinity diagenetic environment.In-situ isotopic analysis shows δ¹³C value ranging from -4.81‰ to 4.29‰,and δ¹⁸O value varying from -7.51‰ to -11.2‰.The δ¹⁸O(SMOW) value of diagentic fluid was calculated to be 5‰ -11‰ by using the fractionation equation of dolomite-fluid oxygen isotope.It is believed based on a comprehensive analysis that the saddle dolomites were formed in a relatively closed deep burial setting and were the products of thermal fluid modification.The saddle dolomites were probably formed in places where pores and fractures were well developed during a recirculation of heated formation water.The high salinity formation water could be the mixture of paleo-seawater during sedimentation period and underground hot brine during burial period.Origin of the saddle dolomite reveals that the saccharoidal dolostones with coarse grains are the result of recrystalization of surface or near surface dolostones.